One significant issue associated with the development of commercially viable solar radiation-based electrical power generating systems of the type described above is energy conversion efficiency.
Energy conversion efficiency is a very strong driver of the economics of solar power systems. When a conversion efficiency increase can be achieved at a percentage system cost increase which is less than the percentage efficiency increase, the commercial prospects for the solar power system are improved.
Current indications are that the present invention can achieve an efficiency increase of the order of 5% at a system cost increase of less than 0.5%.
Two issues that have a negative impact on energy conversion efficiency in a solar radiation receiver are (a) dead space and (b) flux variation.
The issue of dead space relates to the ratio of active photovoltaic cell area versus the total module area. This ratio is the Active Cell Area Ratio (ACAR). Dense array systems known to the applicant typically have ACARs of only 65% to 95%.
Challenges that have limited the ACARs of prior art arrangements known to the applicant include:
(a) a space limitation and complexity of interconnection of adjacent photovoltaic cells to form a circuit;
(b) the ‘bus bar’ connection on the top of cells causing shading of the cells and reducing the effective active area;
(c) the diode connection; and.
(d) the area occupied by an edge seal which provides for safety and reliability
The issue of flux variation relates to the arrangement and characteristics of the cells and modules in a receiver (including cell size and connections between cells) and to the flux distribution impinging on the receiver relating to the collectors for reflecting solar radiation to the receiver. The variation in flux can also be time-based and, typically, is a more significant issue with collectors that reflect and concentrate solar radiation, such as heliostat-based concentration systems.
The applicant has considered the issues of dead space and flux variation in relation to dense array CPV module design.
The applicant decided that it is desirable to have a module that has a high ACAR and a high voltage. The high ACAR results in high conversion efficiency in a module because of the greater percentage of active cell area exposed to radiation for a given module size. This is a driver for larger cells with a minimum number of connectors, diodes and a small edge seal. High voltage allows maximum parallel connections of modules, which results in high average module efficiency across a receiver which has a plurality of modules subject to variable flux. This is a driver for smaller cells. These issues therefore are opposing issues.
Known photovoltaic cells which are used in dense array CPV modules are typically low voltage (0.5 to 6 V). Thus, many cells must be connected in series to achieve a desirable high voltage (typically 200 to 1000 V). This means the cells must be relatively small and there are many connections in the module. This means the connections must consume a minimum of (active area) module space, otherwise the advantage gained will be lost due to the dead space taken up by the increased number of connections.
The many hundreds of cells and connections must also be very reliable or have a means to bypass a failed cell or connection so that the effect due to its failure is minimised.
It has been proposed in the non-patent literature to use a “shingle” arrangement of a string of photovoltaic cells in a module rather than a more standard end-to-end arrangement of cells. The proposed prior art shingle arrangement includes a leading edge of one photovoltaic cell overlapping a trailing edge of a successive cell in a short straight string of cells, to cover an inactive “bus bar” area of these successive cells. The proposed prior art shingle arrangement potentially results in a high ACAR in view of the cell overlap with resultant bad heat transfer.
The applicant has found in research and development work that when cells are appropriately mounted, the shingle approach may be practical for a single short straight length string of cells but may not be a solution for large scale solar radiation-based electrical power generating systems which require large areas of a large number of active cells to be interconnected in one or more lines of cells in order to achieve a required module voltage.
More specifically, limitations/challenges of the known shingle arrangement approach that have been identified by the applicant include:
(a) low voltage due to short string length,
(b) low power due to short string length,
(c) high mechanical strain due to the monolithic nature of overlapping arrangement effectively increasing size of the cell physical structure in one direction,
(d) poor heat transfer due to limited contact of sloped cell with a cooled substrate (such as a copper pad) to which it is usually attached,
(e) difficulties changing direction of a string; and.
(f) cost of interconnects and or modified cells (for non-shingle arrangements).
In summary, there is a need for a shingle arrangement that includes bypass diodes and a simple “cross” connection so that a low strain serpentine string can cover a 2 dimensional array with minimal dead space and excellent heat transfer to minimise cell temperature for use in concentrated light. In more general terms, not confined to shingle arrangements, there is a need for an alternative photovoltaic cell assembly that is suitable for use in a dense array CPV module to the currently available assemblies.
The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.